The effect of reduction and alkylation on the ability of rabbit IgG antibody to activate complement will be studied to correlate structural and functional changes. New, preliminary evidence indicating that an intra-chain disulfide (SS) bond is critical for this effector function of rabbit IgG antibody will be extended to detemine which SS is involved and precisely at what step in the complement reaction sequence the function of reduced, alkylated antibody is impaired. We have found conditions under which the suspected intra-chain SS bond is reduced selectively and other conditions under which it is spared. Samples of IgG antibody will be reduced by each of these methods and alkylated with radioactive ICH2COO bearing different isotypes. These samples will be mixed and subjected to chemical and enzymatic cleavage, followed by isolation and characterization of labelled peptides. The location of each peptide in the known constant region sequence of rabbit IgG will be inferred from its composition and N-terminal amino acid. The double labeling technique will make it possible to distinguish between critical SS bond(s) and those that are reduced coincidentally. Functional studies will include measurements of (1) Cl binding in the absence or presence of antigen in soluble or participating form, (2) Activation of bound Cl, (3) Consumption and utilization of C4 and C2, and (4) Utilization of terminal components of complement in the production of membrane lesions on target cells.